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Complementing Space Telescopes from the Ground

Figure 1. Gemini North Adaptive Optics (AO) image of crowded star field in the bulge of M31. The near-infrared color composite image (J,H,K) was obtained as part of the research by Olsen et al., to study star formation histories in the disk and bulge of M31 (as described in the accompanying press release). The bright star at center is a foreground Milky Way star used as the AO guide star to sample atmospheric turbulence. The image is 20 arcseconds on a side, or roughly the angle spanned by a small grain of sand held at arm’s length.

Image obtained with the Gemini North Telescope on Mauna Kea, Hawai‘i with the ALTAIR AO system built and designed by the National Research Council of Canada’s Herzberg Institute of Astrophysics.

The earth’s atmosphere can block light from astronomical sources. Because of this, space-based facilities such as the Spitzer Space Telescope can obtain unprecedented sensitivity in observations of the infrared universe. Because of its ideal location above our planet’s warm (and absorptive) atmosphere, it can observe very faint infrared (heat) sources.

However, sensitivity is not the only concern for astronomical observations. Ground-based infrared telescopes with much larger apertures provide an excellent complement to Spitzer as they offer significantly higher spatial resolution. This is of particular importance when studying the detailed structure of an object or when observing extremely crowded star fields, such as those near the centers of neighboring galaxies like M31. While once thought to be similar to our own galaxy, during recent years it has become evident that M31 has evolved very differently from the Milky Way as well as sharing some similarities. Astronomers are interested in studying both the Milky Way and M31, for the different perspectives on galaxy evolution that they provide.

Infrared observations using ground-based telescopes have advanced significantly with the implementation of infrared optimization technologies (like the silver coatings used on the Gemini mirrors to increase sensitivity and the use of adaptive optics to compensate for the blurring introduced by the atmosphere). While it is still necessary to be in space for maximum infrared sensitivity, ground-based facilities–with large apertures–play an increasingly important role in exploring the infrared universe.

Two studies featured today at the American Astronomical Society Meeting in Calgary, Canada, bring into focus the core and evolution of our nearest large galaxy neighbor. High-resolution infrared observations of the Andromeda Galaxy (M31) made at Gemini Observatory reveal an intriguing dust-enshrouded star near the core of the galaxy, while extremely sharp adaptive optics images allowed the analysis of thousands of individual stars that indicates a long-stable environment around the galaxy’s core.

These results were announced by two teams of researchers during a press conference at the meeting that also included the release of new images of the entire span of M31 obtained with the Spitzer Space Telescope. The Gemini teams included Canadian researcher Tim Davidge (NRC, Herzberg Institute of Astrophysics, Victoria BC) and Knut Olsen of the National Optical Astronomy Observatory (NOAO). The new data about individual stars in M31 obtained with the Gemini North Telescope on Mauna Kea, Hawaii, demonstrate dramatically the complementarity of ground-based telescopes, which can record images with unprecedented spatial resolution, and space-based facilities, which have higher sensitivity.

The teams used Gemini’s infrared capabilities in order to penetrate obscuring dust to study individual stars near the center of M31 that could not be detected with visible light. Gemini’s resolution allowed the teams to isolate and study individual stars in densely crowded fields in the galaxy.

Figure 2. Near-IR images of the nucleus of M31 obtained at Gemini North and CFHT. The top two panels are reduced data, the bottom panel is a model created by the team. In the model a point-source contributing 20% of the total flux offset 0.1" from the bright nucleus is added to reproduce the morphology seen in the K' observation.

Looking Deep Into the Heart of Andromeda

In the study led by Davidge the core of M31 was observed in the infrared to search for objects that could only be resolved using a large ground-based telescope of Gemini’s aperture (8 meters). This work discovered a compelling infrared source near the galaxy’s core that shares a common link with our Milky Way galaxy.

The core of M31 contains a puzzling dual nucleus – one of which harbors a supermassive black hole that is heavier than the black hole at the heart of our own galaxy. The intense gravitational field from the black hole may tear apart the clouds of gas and dust that are the nurseries of star formation, so it was thought that the center of M31 was only populated only by old stars.

However, the Gemini data contain information about a luminous object called an Asymptotic Giant Branch (AGB) star that lies between the two nuclei. AGB’s are highly evolved stars that spew dust into space and create extended dust shells that emit excess thermal infrared light. Such stars have young ages (due to their short lives), and are also seen near the center of our galaxy.

This finding provides critical information in the ongoing effort to compare the evolutionary paths of M31 and the Milky Way. It suggests that the centers of M31 and the Milky Way may have had broadly similar star-forming histories.

“Now we see that the centers of M31 and the Milky Way may be more similar than once thought," said Davidge. "These two neighbors in space share some similarities, although not where we might expect. This agreement gives us hope that the center of the Milky Way may be representative of other galaxies. If so we can use our home galaxy as a laboratory to understand much more distant galaxies.”

Figure 3. The locations of the NIRI/Altair and HST/NICMOS fields are shown with respect to an optical image mosaic of M31, courtesy of the NOAO Local Group Survey (Massey et al. 2006). The image is roughly 2.5 degrees on a side, or 5 times the diameter of the full moon.

Observations by the team led by Knut Olsen also used Gemini North, but with the adaptive optics (AO) system ALTAIR, developed at the National Research Council of Canada’s Herzberg Institute of Astrophysics in Victoria BC. This work provides the deepest and highest-resolution images ever obtained of the central bulge and inner disk of M31 in the near-infrared (but not including its nucleus).

These AO near-infrared observations isolated thousands of individual stars in three fields within 9 arcminutes (6,500 light-years) of the galaxy’s core. When this information was combined with HST/NICMOS data, the team was able to derive rough star-formation histories for stellar populations in the bulge and inner disk of M31. The analysis shows that most stars in these regions are relatively old, with heavy-element compositions similar to our Sun, regardless of the star’s location relative to the bulge and disk.

According to current theories of galactic formation and evolution, much of the buildup of a galaxy occurs through the merging of smaller clumps of matter into bigger ones. The remainder of a galaxy’s mass is acquired by slower accretion from surrounding space. Galaxy disks are relatively fragile, and are destroyed in overly violent mergers. Measuring the ages and abundances of the majority population of stars in galaxy disks thus tells us when this merger activity must have settled down. The new results suggest that the disk that we currently see in M31 has been around for at least 6 billion years, or roughly half the age of the universe, and could have existed relatively undisturbed at even older ages.

The results were obtained by comparing the observed distributions of the brightness of individual stars with knowledge of how the luminosities of stars evolve with time for stars with different masses and chemical compositions. A complicating factor is that interstellar dust also affects the observed brightness of stars. For example, the Spitzer image of M31 shows that there is plenty of dust in the galaxy. Observing in the near-infrared, as opposed to optical wavelengths, limits the effects of dust on the results.

Still deeper observations of more fields in M31’s disk are needed to illuminate its star formation history in even greater detail. Large AO-equipped ground-based telescopes provide the high spatial resolution that is the key to seeing an unblended view of the vast numbers of stars present at any location in M31.
“One of the limitations of our study was that in order to use adaptive optics, we had to select fields with a relatively bright foreground guide star nearby,” said Olsen. “Now that Gemini and other large ground-based telescopes are getting equipped with laser guide stars we’ll be able to probe almost any location in the galaxy with adaptive optics and untangle the blur and overlapping of star images to really understand the full story of what these stars can tell us.”

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Maunakea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Científica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.